1. Field of the Invention
The present invention relates to a power supply for measuring the line impedance of an underground cable, and more particularly to a power supply for measuring the line impedance of an underground cable comprising an input terminal section including a plurality of input terminals connected to a power source; a plurality of autotransformers connected respectively to the input terminals of the input terminal section; a transformer adjustment tap connected to all of the autotransformers; a plurality of 3rd-harmonics cancellation transformers connected respectively to the autotransformers; and an output terminal section including a plurality of output terminals connected respectively to the 3rd-harmonics cancellation transformers, whereby the autotransformers are simultaneously controlled by the transformer adjustment tap to equally transform power supply voltages supplied respectively to the autotransformers, and 3rd-harmonics of the powers are cancelled in the 3rd-harmonics cancellation transformers to remove noise, so that the powers are applied in a stable manner respectively to the phases of the underground cable conductor.
2. Description of the Related Art
In order to measure the line impedance of an underground cable, power is applied individually to each of the A, B and C phases of the underground cable conductor, and the voltage, current and power factor of each power source applied is measured. The measured value is then calculated while classifying the underground cable into a positive-phase-sequence circuit and a zero-phase-sequence circuit, so as to calculate the positive-phase-sequence component impedance and the zero-phase-sequence component impedance of the underground cable.
In the prior art, power is boosted at a predetermined transformation rate using three single-phase transformers and three single-phase slidacs (or variable transformers), and the boosted power is then applied individually to each phase of an underground cable conductor, so as to measure the impedance of the underground cable. This conventional impedance measurement method has the following problems. First, it is very difficult to boost the power, which is applied respectively to the phases of an underground cable conductor, at the same transformation rate. In addition, since the measurement is affected by the third harmonics, large errors occur in the measured impedance values. Further, there are risks that an abrupt voltage variance may cause a cut in power of the underground cable line or breakage of impedance measurement equipment.